The general field of this invention relates to processes for concentrating ethanol from aqueous solutions thereof, such as fermentation beer, to obtain an ethanol concentrate which can be used as a motor fuel, or mixed with gasoline to produce a motor fuel, such as gasohol. More specifically, the field of this invention relates to adsorption processes for concentrating ethanol from aqueous solutions thereof.
Ethanol can be readily produced by fermentation processes, which yield dilute aqueous solutions of ethanol. However, the concentration of ethanol from aqueous solutions by the traditional distillation processes used for producing beverage grade ethyl alcohol requires large amounts of heat energy. Beverage grade ethyl alcohol is usually produced as the azeotrope containing about 5% water by weight. It has been estimated that the production of this azeotrope by distillation requires about 1.4 to 1.6 times more energy than will be liberated when the ethanol is burned as a fuel. The cost of growing and gathering the biomass for the fermentation is additional, and this cost is higher where an expensive cereal grain is used such as corn. Further, for some motor fuel uses, such as in producing gasohol, the ethanol must be substantially anhydrous, and therefore a further azeotropic distillation requires as much energy as the fuel value of the alcohol. See Hartline, "Lowering the Cost of Alcohol", Science, Vol. 206, 41-42 (1979).
The Hartline article cited above reviews various attempts to develop lower cost processes for concentrating ethanol from fermentation beers. The only adsorption process mentioned in this article refers to experiments with the use of zeolites as molecular sieves to selectively adsorb water from aqueous ethanol, thereby increasing the concentration of the residual solution. It is stated that a promising candidate for such a process is clinoptilolite, which is a naturally occurring zeolite, and the article also referred to tests with synthetic zeolites for the same purpose being conducted by the Linde Division of Union Carbide Corporation.
It is known that silicalite and similar crystalline silica polymorphs are capable of removing small molecule organic compounds from dilute aqueous solutions, such as wastewater contaminated with organics, which need to be removed to reduce the BOD of the water. See U.S. Pat. Nos. 4,061,724 and 4,073,865, and Flanigen et al, "Silicalite A New Hydrophobic Crystalline Silica Molecular Sieve" Nature, Vol. 271, pp 512-516, (1978). Although silicalite is commercially available from the Linde Division of Union Carbide Corporation, prior to the present invention its use has not been suggested for concentrating ethanol in fermentation beer to obtain a highly concentrated ethanol product.